The present invention relates generally to the transfer of images, patterns or designs onto a decorative medium, and more specifically to the transfer of a seamless holographic pattern onto a decorative medium such as, for example, a polyethylene (PET) film or other carrier (before or after metallization), or on a foil or other medium without a carrier. Although not limited thereto, the present invention has particular utility and significance in micro-embossing applications such as, but not limited to, holographic transfers where a surface seems essentially flat, yet contains minute grooves to facilitate the reflection of light. These grooves are typically only about one-quarter of micron in depth and their integrity must be maintained as best as possible on a die in order to effect an adequate transfer onto a decorative medium such as foil. Since groove depth is necessarily limited and often critical, flaws cannot be tolerated in reproduction of holographic patterns. The present invention addresses these needs.
Holographic images, patterns or designs are transferred or micro-embossed onto a web or length of material (for instance, a decorative foil on a carrier web) by a roller which carries on its outer cylindrical surface a shim having the holographic image, pattern or design. Heat and pressure are used to micro-emboss the hologram on the shim from the roller to the web or length of decorative material. This micro-embossing process is conventional. The shim which is wrapped around the roller is established in planar form by a micro-embossing operation by which a small nickel shim (typically 2 inches by 2 inches) which carries the hologram is attached to a stamp, and the hologram is micro-embossed into a planar plastic sheet by a step and repeat process. To facilitate this step and repeat operation, the planar stamping surface is indexed linearly in the X and Y directions across the planar plastic sheet until the micro-embossing is completed on the entire planar surface. The sheet is then sprayed with a silver conductive spray, and subsequently placed in an electro-plating bath to form a durable nickel shim (an electroforming process). This nickel shim is removed from the plastic sheet and is wrapped around a cylinder to form a cylindrical embossing die. In addition to the long and involved process to make the nickel shim, once the nickel shim is wrapped around the cylinder, the ends of the nickel shim form a side-to-side break in the holographic pattern so that the resulting holographic foil includes a production seam made after each revolution of the cylinder. It is also noted that there will also be slight "recombining" seams created by "recombining" the design by the step and repeat process. These recombining seams are usually insignificant since they are either different to see with the naked eye and/or are incorporated into the overall design on the decorative medium. These recombining seams are not eliminated by the present invention.
U.S. Pat. Nos. 4,790,893 and 4,968,370 both relate to the replication of information carriers such as compact discs. The master for replicating the information carriers is a planar nickel shim with patterned or image surface depressions or pits corresponding to audio or video recorded digital information retrievable by, for instance, laser scanning. This planar nickel shim master is wrapped partially around a cylinder and is embossed onto an endless web of a thermoplastic or other material used as the base for the compact disc or other information carrier. This is similar to the above described process in that a planar shim is partially wrapped around a cylinder for embossing onto a web of material. Again, seams will appear in the web of material, but in the replication of information carriers such as compact discs these seams do not form part of the resulting product, and thus do not create a problem as with decorative foil.
U.S. Pat. No. 4,923,572 is directed to a cylindrical embossing tool which can be used for embossing a web of material without leaving seams. Described in this patent is a complex method of making a shim (in the form of a tube or sleeve) which carries an imaged electroform and can be placed over a carrier cylinder by introducing air into the interface of the tubular and floating the tubular shim into position to form a supported embossing tool. In the alternative, the tubular shim carrying the imaged electroform can be supported by a number of rollers to form an endless belt embossing tool. Significantly, however, the electroform embossing tool, whether an endless belt embossing tool or cylindrical embossing tool, is formed by first stamping a polymeric or thermoplastic embossable material layer on a cylinder with a stamper which carries an image or pattern on a concave-shaped stamping surface. A thin layer of metal such as silver could also be deposited prior to embossing the embossable material layer to render it electrically conductive and/or optically reflective. A nickel electroform is then electroformed on the embossable material layer on the cylinder, which nickel electroform carries a negative of the stamped image or pattern. A reinforcement layer in the form of an adhesive, resin or fiberglass particles is then provided to mask the nickel electroform and provide stability and rigidity to the composite layers. These composite layers are then removed from the cylinder, and then the reinforcement together with the nickel electroform are removed from the composite layers. The inside of the hollow cylinder having the nickel electroform is then electroplated to provide another electroform which, by virtue of the negative on the nickel electroform, carries the stamped image or pattern. That second electroform is then removed and either placed over a cylinder or between rollers as described above. The result of this intricate process is a cylindrical embossing tool or a belt embossing tool which can emboss an image or a pattern onto material without leaving seams after each revolution of the cylinder or the belt. However, in addition to the intricacy required to prepare the cylindrical embossing tool or the belt embossing tool, there may be problems with the strength or the durability of the second electroform.
A cylindrical embossing die having a relatively simple pattern burnished into the nickel plating on a steel cylinder has also been used to transfer holograms onto decorative foils in a seamless manner. However, the cylindrical embossing die was produced by an engine-turning operation using an ultra precision machining device which employs, for instance, a single crystal diamond cutting tool in a lathe-type machining process. The operation is intricate and expensive and, more importantly, is limited to extremely simple geometric patterns which can be established by such a lathe-type machining process. The only patterns known to have been established on a nickel plated cylinder by this engine-turning operation is the so-called "laser" pattern which is an extremely simple pattern. Such an operation cannot be used to establish an intricate geometric pattern on a cylinder for use in embossing a seamless pattern on, for instance, metallized PET film.
It is thus apparent that an improved method and die for effecting the seamless transfer of an image, pattern or design onto a material is warranted. Such an improvement should address the cost in manufacturing the apparatus, the durability of the die and the scope of the method in establishing images, patterns or designs for seamless transfer.